This invention relates to a modified indirect gas heated retorting process for retorting hydrocarbonaceous solids. More specifically, a relatively small amount of oxygen is combined with retort produced recycle gas after the recycle gas has been heated, but before it is flowed into the retort.
As used herein, the term retorting refers only to the injection of a gas into a vessel containing crushed mined oil shale, coal or tar sands to thermally convert or pyrolyze the organic hydrocarbonaceous matter in these normally solid hydrcarbonaceous materials at temperatures in excess of 600.degree. F. (315.degree. C.) to gases and oil mist or droplets. The retort vessel will normally be above-ground, but it might be placed or formed in a hole or tunnel in the ground near the surface of the earth. Such term, for example, does not include in situ retorting processes, retorting processes relying on heat transfer between hot heat carrying solids and the hydrocarbonaceous solids being retorted, or liquefaction processes using heated liquids or slurries.
Retorting processes using gases to heat and convert normally solid hydrocarbonaceous matter are generally classified as being either a combustion retorting process, or an indirect heated retorting process, or a combination combustion-indirect heated retorting process. In the combustion process, oxygen (e.g., air, oxygen with steam, oxygen with carbon dioxide, etc.) is injected at one or more points into a bed of hydrocarbonaceous solids to burn hydrocarbonaceous matter in the solids. In the combustion process, this provides the heat for retorting the solids. In the indirect heated process, recycle gas derived from the retort is heated in a separate furnace. The heated recycle gas is then injected at one or more points into a bed of hydrocarbonaceous solids. The heat content of the recycle gas provides the heat for retorting the solids. In the combination process, heated recycle gas is injected into a bed of solids while an oxygen containing combustion gas is injected at another point into the solids after the solids have already been partially pyrolized by the heated recycle gas. The oxygen burns hydrocarbonaceous matter remaining after the solids have been retorted by heated recycle gas. These gas retorting processes may be used in batch or continuous fashion and many types of retort vessels have been proposed. For example, U.S. Pat. No. 3,361,644 describes an indirect heated process wherein heated recycle gas is flowed downward through a bed of solids while a rock pump pushes crushed mined hydrocarbonaceous solids upward through a vertical retort. U.S. Pat. No. 3,841,992 describes an indirect heated process wherein the hydrocarbonaceous solids are fed downward through a vertical retort while heated recycle gas is injected at two central points. Revolving and traveling grate-type retorts may also be adapted to the indirect heated recycle gas retorting process.
In order to illustrate the relationship between objectives of this invention and the prior art gas retorting processes, a typical gas retorting process will be considered as having a final preheating zone, a final pyrolysis zone, and a spent solids cooling zone. Actually, in a vertical retort, the preheating and pyrolysis zones are not distinct. For this illustration, several interrelated conditions will be mentioned. In some respects, these conditions depend on the type of retort. For example, the rock pump, upwardly fed vertical retort has considerations involving similar theories, but which are twisted around or act differently. For sake of simplicity, this description of the prior art concerns will generally be limited to the vertical retorts wherein the solids flow downwardly while the gases flow upwardly.
In a gas retorting process, gas flows through a bed of solids. It is desirable to keep the rate and total amount of gas flow at the minimum required for retorting. Increasing the rate of gas flow causes a number of problems. The solids in the bed are not uniform and at higher rates, there is greater channeling. This results in insufficient heat distribution. At higher rates, solid particles are entrained in the gas. These particles can plug the bed and increase channeling. In addition, entrained particles which are not left in the bed contaminate liquid products obtained from the retort effluents. These contaminants are difficult to remove without loss of valuable product. In the pyrolysis zone of the retort, gases and oil mist or droplets are produced. The bed of solids has some aspects of a mechanical separator and at higher gas rates, there is both a condensation-reflux effect and effects of droplet enlarging and striking the solids at sufficient force to stick and be carried back into the pyrolysis zone. The rate of gas flow also affects size and nature of separate equipment for treating the retort effluents and for handling the gas to the retort.
As previously mentioned, the retort produces a gas. It is desirable that the gas have as high a BTU content as is feasible. Combustion retort processes, especially those using air, produce lower BTU content gas than the indirect heated process. From this and other standpoints, the indirect heated process is preferred. But the indirect heated process has disadvantages. The heat for pyrolysis comes from the heat content of recycle gas which is heated in a separate furnace. For this discussion, it is assumed that heat content of the heated recycle gas is dependent on its temperature and specific heat. The specific heat of the gas is small in comparison to the specific heat of the solids in the retort. At the same time, there are limits to which the recycle gas can be heated in the furnace. Furnaces are relatively inefficient. The furnace must be operated at a higher temperature than the recycle gas. The furnace residence time affects the degree of heat transfer. The recycle gas contains hydrocarbons. At the temperatures required for standard indirect heated processes, coking or carbonization of the recycle gas is a problem. Coke fouls the furnace tubes decreasing heat transfer efficiency and creating plugging problems in the furnace and in piping leading from the furnace to the retort. When the temperature is lowered to reduce coking, the amount of heated gas needed to supply heat for the retort goes up. This increases the flow rate of gas in the retort, creating the problems previously mentioned. This can also affect the rate at which the retort can handle solids.
In some indirect heated retorts, after leaving the pyrolysis zone, the spent or pyrolyzed solids are cooled by passing an unheated portion of the retort effluent gas through the spent solids. When this cooling gas reaches the pyrolysis zone, it comingles with the incoming heated recycle gas from the furnace and the gas flow rate in the pyrolysis and preheat parts of the retort is dependent on the total flow of these two gas streams and the gases generated by pyrolysis. If the cooling gas is at a lower temperature than the pyrolysis zone, there will be added heat burdens on the heated recycle gas. This increases the total amount of heated recycle gas that is injected into the retort which as previously mentioned in undesirable. If the flow rate of the cooling gas is reduced so that when the gas reaches the pyrolysis zone, it will have nearly the same temperature as the pyrolysis zone, the exit temperature of the spent solids from the retort is too high. In general, the process is designed to balance the two rates of gas flow in a way that maximizes thermal efficiency without causing other problems. In some processes, the spent solids are taken from the retort without cooling, but this lowers the thermal efficiency of the overall process and requires water for quenching the solids. In areas where hydrocarbonaceous deposits are found, water is frequently scarce. If the water is recovered and recycled, additional water treating equipment is required. Even when water is more abundant, there is a water disposal problem.
The combination indirect heated-combustion process was proposed as a compromise between the easier to operate combustion retort and the product advantages of the indirect heated process. In this combined process, in order to avoid burning valuable products, the externally heated recycle gas is injected into the retort at a point ahead of the combustion gas. In other words, the combustion portion of the retort burns hydrocarbonaceous matter left after the solids have been partially retorted by the heated recycle gas. The combined process has disadvantages. The retort effluent gas has a lower BTU value than the gas by the indirect heated process, especially if air is used. In addition, if unheated retort effluent recycle gas is used to cool the spent solids, the combustion part of the retort consumes the hydrocarbons in the recycle gas. If inert gases are used to cool the solids, the BTU value of the gas product is lowered. In comparison to the indirect heated process, combustion of the residual hydrocarbonaceous matter increases the amount of cooling gas that is required and increases gas flow rates in the retort. Combustion also increases the decomposition of carbonates in the inorganic mineral matter in the solids. For example, in a test at 1100.degree. F. (593.degree. C.), the percent of decomposition of the carbonate was 25.6 percent by weight while at 950.degree. F. (510.degree. C.), the percent of decomposition was 2.6%. Energywise the higher degree of decomposition is equivalent to using 27 pounds of oxygen per ton (13.5 grams per kilogram) of feed. In other words, higher gas flows are required. Combustion increases the disintegration of the crushed solids. It is, therefore, desirable to minimize the amount of combustion in the retort provided that there is not an offsetting loss in other parts of the retort facilities.
The foregoing discussion of gas retorting processes illustrates some of the difficulties in fine tuning process. In addition, this shows some of the reasons why the processes are not flexible to inherent changes in conditions. For example, there are inherent variations in the hydrocarbon content or richness of the material to be retorted, in the flow and size of the solids, in ambient temperature, and in the furnace for heating the recycle gas. It is, therefore, desirable to provide a retorting process that provides better control or response to the conditions mentioned.